Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!uwm.edu!rpi!crdgw1!sunne!brownpc
From: brownpc@sunne.crd.ge.com (Paul C Brown)
Newsgroups: comp.std.c++
Subject: Re: the most dissatisfying part of c++
Message-ID: <10656@crdgw1.crd.ge.com>
Date: 1 Aug 90 21:02:00 GMT
References: <27634@netnews.upenn.edu>
Sender: news@crdgw1.crd.ge.com
Reply-To: brownpc@sunne.crd.ge.com (Paul C Brown)
Lines: 58

In article <27634@netnews.upenn.edu>, limsoon@saul.cis.upenn.edu
(Limsoon Wong) writes:
|>
|>the program below is a contrived one.   it cannot  be compiled.
|>the reason is a type fault. however, it exhibits a very central
|>characteristic of update.    something should be  done to allow
|>this kinds of programs.	   			    ---limsoon.
|>
|>
|>
|>#include <stream.h>
|>class a { 
|>  public: 
|>    a& test() { 
|>	// do some modification to the state of this object,
|>	// then ...
|>	return *this;
|>};
|>class b : public a { 
|>  public:
|>    int b;
|>};
|>main() { 
|>  b B; 
|>  B = B.test();
|>  // type violation despite the fact that 
|>  // `B' and `B.test()' are the same object.
|>}
 
Your proposed program is not "type" correct. The method "test" returns
an object
of type a, not of type b. Objects of type a do not exhibit all of the
properties of type b, and therefore are not valid values for variables
of type b. The 
following version of main is type correct:

main() {
  b B;
  a A;
  A = B.test();
};

To accomplish what you were trying to do would require a second method on 
class b, also named test, that returns an object of type b. If the original
method on class a does, indeed, simply modify the referenced instance, then
the new method could be as simple as:

b::test() {
  a temp;
  temp = this.test();
  return *this;
};

As you have observed, strong typing does not give you everything for
free!                         
Paul C. Brown				brownpc@crd.ge.com
GE Corporate Research & Development
Schenectady, New York